1. Field of the Invention
The invention relates to mask repair and particularly to performing mask repair using a multiple exposure technique.
2. Discussion of the Related Art
In designing an integrated circuit (IC), engineers can use computer simulation tools to help create a circuit design. The circuit design consists of individual devices coupled together to provide certain functions. To fabricate this circuit in a semiconductor substrate on a wafer, the circuit design is translated into a layout. Computer aided design (CAD) tools can assist layout designers in the task of translating the discrete circuit elements (such as gate electrodes, field oxidation regions, diffusion regions, metal interconnections, etc.) into shapes in the layout that implement these circuit elements.
The layout can be transferred onto the semiconductor substrate using optical lithography. Specifically, for each layer of the circuit design, a radiation (e.g. light) source is shone on a mask (wherein the term mask can also refer herein to a reticle) corresponding to that layer. This radiation passes through clear, e.g. quartz, regions of the mask and is blocked by opaque, e.g. chrome, regions of the mask, thereby selectively exposing a photoresist layer on the wafer. The photoresist layer can then be developed and the semiconductor layer underlying the photoresist can be etched, thereby creating a pattern that defines the shapes of that layer. This process can be repeated for each layer of the circuit design.
As circuit designs become more complicated, it becomes increasingly important that the masks used in photolithography can accurately transfer the layout to the wafer. Unfortunately, the machines used to manufacture these masks cannot do so without error. In fact, in a typical manufacturing process, some mask defects do occur outside the controlled process.
A defect on a mask is anything that is different from the circuit design and is deemed unacceptable by an inspection tool or an inspection engineer. If there are no defects, or defects are detected but determined to be within tolerances set by the manufacturer or end-user, then the mask can be used to expose a wafer. However, if defects are detected that fall outside tolerances, then the mask fails the inspection, and a decision must be made as to whether the mask can be repaired to correct the defects, or whether the defects are so severe that a new mask must be manufactured. This process is continued until a manufactured mask passes the inspection.
The repair of an area on a mask that is erroneously covered with chrome entails the removal of such chrome. Three types of repair tools can be used to repair a mask with chrome defects: a focused ion beam tool, an optical tool, or a micro-chisel tool. Of these tools, the focused ion beam is most frequently used because it provides the best imaging capability of the defect area as well as the best accuracy due to its small spot size. Unfortunately, as a result of these features, a repair of each defect using the focused ion beam may take up to an hour. As the mask itself takes approximately twelve hours to make, the decision to repair versus making a new mask can be contingent on the number of defects identified. For example, if ten defects falling outside tolerances are identified on the mask, then a mask shop may decide to manufacture a new mask rather than try to repair the mask.
Of importance, trying to repair a mask does not guarantee that the defects can be eliminated. Specifically, stopping a repair process too soon can leave residual chrome on the mask, whereas stopping a repair process too late can cause phase or transmission defects on the mask. For example, during a repair operation using the above-described tools, some portion of the exposed quartz surface could be removed, thereby resulting in an undesirable difference of phase between the “repaired” area and its surrounding area. Both phase and transmission defects can adversely affect feature printing on the wafer.
Therefore, a need arises for a commercially viable technique that can successfully repair a mask without causing phase and/or transmission defects.